class_docstr: Base class for testing FXRateReader implementations.
  
  To test a new FXRateReader implementation, subclass from this base class
  and implement the ``reader`` property, returning an FXRateReader that uses
  the data stored in ``cls.fx_rates``.
code: def reader(self):
      raise NotImplementedError("Must be implemented by test suite.")
  
imports: import itertools
  import pandas
  import numpy
  from zipline.data.fx import DEFAULT_FX_RATE
  from zipline.testing.predicates import assert_equal
  import zipline.testing.fixtures
label_desc: Use image node_id 2 for calling the _FXReaderTestCase obj's underlying member method code with example usage: obj.reader() without return types